Where is the ac pressure switch on a Cat 304C?

[4lane]

AC is not working on my Cat 304C CR. Compressor spins freely, cab switch is good, no power at compressor. I’m looking for the AC pressure switch but can’t find the damned thing by tracing the lines. Help?!

 

[4lane]

I don’t see it in this system. I did find this 3-pin ‘relay’ which was located in the most inaccessible spot. How can I check to see if this thing is good or bad?

 

[Nige]

Five versions of 304C, and apparently some differences between them.
A Serial Number would likely get you the answer.

 

[4lane]

Serial is: CAT0304CHFPK03040

I only see the following components in the AC system. I believe, the thermostat shown here being a thermistor and the diode is the relay I’d like to test.

 

[Nige]

The pressure switch is on the schematic as thermostat switch 272-7875. This services to 283-4171 which is part of the A/C compressor 283-4170 and is not sold separately according to what I can see. Are there any electrical connections on the compressor other than the one for the clutch.?

 

[heymccall]

Looks like a switch in the compressor body.

 

[4lane]

The pressure switch is on the schematic as thermostat switch 272-7875. This services to 283-4171 which is part of the A/C compressor 283-4170 and is not sold separately according to what I can see. Are there any electrical connections on the compressor other than the one for the clutch.?

Yes, just as heymccall shows, there is an electrical connection to a switch in the compressor. Are you saying this is the low pressure switch but it's not sold separately?

 

[Cmark]

You're barking up the wrong tree here gentlemen. That switch on the compressor is a thermal cutout. The pressure switch appears to be in the A/C box inside the cab.

 

[4lane]

You're barking up the wrong tree here gentlemen. That switch on the compressor is a thermal cutout. The pressure switch appears to be in the A/C box inside the cab.

I had moved my focus to the AC box in the cab, too. I found a thermistor, and the compressor circuit board relay which I wanted to test in my earlier comment. But in your diagram I see a low pressure switch and a temperature sensor (evaporator), neither of which I found, but I do see this - not sure what it is - temp sensor?

 

[Cmark]

That's your pressure switch. What colour are the wires? Do they match the schematic?
The evap temp sensor will be a wire type thing either clipped to the core or stuck into the fins.

 

[4lane]

That's your pressure switch. What colour are the wires? Do they match the schematic?
The evap temp sensor will be a wire type thing either clipped to the core or stuck into the fins.

Wire colors don’t entirely match with the schematic, but it seems you are correct. The image above is the low pressure sensor, the evap temp sensor (or thermistor) and the two wire that goes into the relay I pictured are the same color. That relay has a two wire and a three pin to a circuit board. I thought this was the AC relay, but the only thing with a 5 wire is the “Relay AS (Blower Motor)”. Since it’s tied to the temp sensor this must fluctuate the blower as the temp changes in the cab. Make sense?

That leaves me with finding the compressor relay which looks to be a standard 4 pin relay. I just can’t find where it’s located. Nor can I find the part number for the low pressure switch. Anybody?

 

[Cmark]

Typically, the evap temperature sensor will control the A/C by cycling the compressor. The pressure switch will completely cut power to the compressor in the event of over-pressure or under-pressure in the system. Neither will have any effect on the blower speed. Indeed, if you follow the wiring the "Relay as (blower motor)" isn't connected to the blower circuit.

It looks to me like the "Relay as (blower motor)" has been mis-labelled. It should be "Relay as (A/C compressor)".

 

[4lane]

Typically, the evap temperature sensor will control the A/C by cycling the compressor. The pressure switch will completely cut power to the compressor in the event of over-pressure or under-pressure in the system. Neither will have any effect on the blower speed. Indeed, if you follow the wiring the "Relay as (blower motor)" isn't connected to the blower circuit.

It looks to me like the "Relay as (blower motor)" has been mis-labelled. It should be "Relay as (A/C compressor)".

On the schematic there is already dedicated "Compressor Relay". The "Relay AS (Blower Motor)" I'm still unsure of what it does. It's labeled "AS" in all Cat parts so it doesn't seem like it's mislabeled, and I see it as tied into the blower motor circuit. Here it is in the AC system:

 

[Cmark]

OK, Let's make it clear. First, "AS" is Caterpillar shorthand for "assembly".

The "relay AS (blower motor)" takes inputs from the "pressure switch (refrigerant)" and "temperature sensor (evaporator)". It isn't tied into the blower motor circuit in any way which can control the blower.

If the pressure switch is closed AND the temperature sensor is above the set point, the "Relay AS (blower motor)" then turns on the "compressor relay". This then turns on the compressor.

 

[4lane]

OK, Let's make it clear. First, "AS" is Caterpillar shorthand for "assembly".

The "relay AS (blower motor)" takes inputs from the "pressure switch (refrigerant)" and "temperature sensor (evaporator)". It isn't tied into the blower motor circuit in any way which can control the blower.



If the pressure switch is closed AND the temperature sensor is above the set point, the "Relay AS (blower motor)" then turns on the "compressor relay". This then turns on the compressor.

Thank you for that clarity. Now back to my original question, how can I effectively test the Relay AS (Blower Motor) to see if it's faulty or not?

 

[Cmark]

Ignition on.
Blower switch in any position except off.
A/C switch on.
You should have +12v to pin 4.
You should have ground to pin 5.
The Relay AS (blower motor) will turn on ground to pin 3 if the evap temp sensor is warm.

 

[4lane]

Ignition on.
Blower switch in any position except off.
A/C switch on.
You should have +12v to pin 4.
You should have ground to pin 5.
The Relay AS (blower motor) will turn on ground to pin 3 if the evap temp sensor is warm.

Extremely helpful and very thankful for this level of detail. Getting a multimeter lead on a pin will be super challenging. These components all sit under/behind the bottom of the A/C panel. I had to take the entire thing apart to pull it out, and flip it upside down to get to the Relay AS (blower motor) relay disconnected.

 

[4lane]

How can I isolate testing of the pressure switch and temperature sensor?

 

[4lane]

Ignition on.
Blower switch in any position except off.
A/C switch on.
You should have +12v to pin 4.
You should have ground to pin 5.
The Relay AS (blower motor) will turn on ground to pin 3 if the evap temp sensor is warm.

I did some tests today. Getting 12v to pin 4, ground to pin 5, ground to pin 3 ( it was 80 degrees out. Since the evaporator temp sensor is new, and the Relay AS Blower Motor passed these tests, and the AC switch sent power to this relay, I focused on the compressor relay and low pressure sensor. Since the low pressure sensor was right in front of me I took the opportunity and jumped it for a min. Compressor came on, shut off when the AC switch was turned off, and when on/off with the proper blower speed so compressor relay is good, and compressor at least cycles. In that minute, I didn’t feel much cold air and tubes were not cold. So, I’ve eliminated the electrical components.

That leads me to explore two scenarios: low pressure sensor is bad or system is low on refrigerant. I’m thinking the latter so I’ll get some gauges on it and check the static pressure to see if it’s just a matter of needing refrigerant and go from there.

 

[jbernielh]

I don't know how far apart you have the housing for the blower motor and heater and AC exchangers but I've had to completely dismantle my 304C CR twice now because the condensation on the AC exchanger mixes with the fine dust and dirt going through the system and after a while the AC exchanger (not the heater core it's never been a problem) looks like it's been stucco'd over... super clean and hot water and blow dry and good to go... my indicator is the air flow out the ducting becomes less and less but it happens very slowly over time so you don't really notice.. until it's cleaned out and you realize how much it was obstructed..
I realize this is probably not your issue I was just going to suggest it might be worth looking at or doing while you've got it opened up..

Bernie